1. Field of the Invention
The present invention relates generally to a photosensitive glass paste which is cured by exposure to light, particularly for making an insulation layer in multilayered electronic device, and to a method for manufacturing a multilayered interconnected circuit board using the glass paste.
2. Description of the Related Art
Conventionally, semiconductive elements, such as ICs, etc., are used mounted on a printed circuit board in which an insulating layer having a through hole is formed by processes such as printing processes, etc., on a glass epoxy resin board having fine wiring on the surface thereof.
In recent years, as the demand for high integration, fine interconnection, high speed transmission, high frequency and rapid heat dissipation increases for semiconductor elements, the demand for a printed circuit board capable of accommodating such needs has also increased.
The conventional printed circuit board, however, does not have satisfactory characteristics regarding workability, platability of through holes, adhesive property among layers, etc., resulting in problems. Also, there is a problem of thermal deformation at high temperatures in the above described glass epoxy resin board and there is a limit to increases in density.
In view of the above, a ceramic substrate which uses a ceramic having a high mechanical strength and a high thermal resistance, such as alumina, is considered to be desirable.
As a ceramic substrate, there may be mentioned a printed substrate having fine wiring on the surface thereof and having an insulating layer with through holes formed by the printing techniques, etc., on the surface thereof. In this type of printed substrate, it may sometimes be necessary to form fine through holes in order to accommodate high density packaging.
Also, in recent years, multilayer-type inductors capacitors, etc., formed by laminating a ceramic green sheet provided with electrodes (coil patterns) are being increasingly used. As the level of integration of electronic circuits increases, passive devices such as inductors, capacitors, etc., are also required to be miniaturized.
A spiral multilayer coil capable of yielding high inductance, for example, is manufactured by repeatedly forming an insulating layer, with via holes, on a ceramic green sheet (alumina green sheet) provided with a coil pattern, filling a conductor into the via hole, stacking another ceramic green sheet on the insulating layer, and connecting the coil pattern of the first layer and the coil pattern of the second layer via the via holes. When a coil is manufactured using this process, it is necessary to form fine through holes in order to achieve miniaturization of the product.
In response to the trends to higher density and miniaturization of the multilayer circuit components, a method of forming an insulating layer by using a photosensitive glass paste has been proposed, by which fine via holes which have been difficult to form by conventional printing techniques can be formed.
In this method, a photosensitive glass paste is applied on the entire surface of the substrate by way of screen-printing, etc. The applied photosensitive glass paste is dried, is then exposed and developed through a photomask, and is fired so as to form the fine via holes.
It is to be noted that, the photosensitive glass paste used in this method is a mixture of a photosensitive organic material and an inorganic mixture containing a glass or both a glass and a ceramic. As the inorganic mixture, a mixture in which ceramic material such as quartz, cordierite, alumina, zirconia, mullite, spinel, forsterite and silica are added to the glass at a predetermined proportion is generally used. As the photosensitive organic material, one including a polymer having a functional group in the side chain thereof, a photoreactive compound (monomer), a photopolymerization initiator, a solvent, etc., is generally used.
The conventional photosensitive glass paste, however, usually contains only one kind of glass component having a relatively low glass softening point. Thus, viscous flow is initiated at a low temperature, resulting in a large shrinkage ratio due to sintering and the diameter of the formed via hole will be significantly larger than the diameter after development. Another problem is that during the step of firing, the amount of Ag from the conductive leads made of Ag which constitute the circuit diffusing into the glass is large, degrading insulating characteristics between the layers.
In contrast, when the glass component of the photosensitive glass paste includes only a high melting point glass, as the photosensitive glass paste is treated with heat in the range of the optimum firing temperature so as not to cause deformation of conductive leads made of Ag and Cu, there are problems of insufficient sintering and degradation of insulative properties.
Japanese Unexamined Patent Application Publication 10-120432 discloses a photosensitive glass paste for use in plasma displays, in which a high melting point glass is added to a low melting point glass. Because the content of the low melting point glass is high (40 to 97 percent by weight), when this photosensitive glass paste is employed in a multilayered interconnected circuit board, deformations of via holes due to shrinkage cannot be avoided. Also, because viscous flow of the low temperature glass occurs at low temperatures, a conductor such as Ag will diffuse, causing the insulation properties between layers to degrade. Furthermore, because the functional group, such as carboxyl group, in the photosensitive organic component reacts with polyvalent metal such as boron contained in the glass component, the polymer chains are cross-linked and the viscosity of the paste is abnormally increased, resulting in gelation.
Accordingly, it is an object of the present invention to provide a photosensitive glass paste which is not susceptible to gelation, shrinkage due to sintering and diffusion of conductor compositions such as Ag, by which a glass layer having via holes of precise predetermined shape and size can be formed at predetermined positions. Another object of the present invention is to provide a method for manufacturing a multilayered interconnected circuit board using the photosensitive glass paste.
In order to achieve the above objects, a photosensitive glass paste is provided comprising an inorganic component including a glass powder, and a photosensitive organic component, in which the glass powder comprises (a) about 1 to 30 percent by weight of a low melting point glass powder having a glass softening point in a range of about 400xc2x0 C. to 600xc2x0 C. and (b) about 70 to 99 percent by weight of a high melting point glass powder having a glass softening point about 300xc2x0 C. or more higher than the softening point of the low melting point glass powder.
Because the glass component which constitutes the main portion of the inorganic component of the photosensitive glass paste of the present invention contains a high melting point glass as the main component thereof and limits the proportion of the low melting point glass to be less, shrinkage due to sintering, diffusion of conductive components such as Ag, and gelation can be prevented.
Also, by using this photosensitive glass paste, for example, a glass layer having via holes of desired size can be reliably formed.
The photosensitive glass paste of the present invention uses about 1 to 30 percent by weight of a low melting point glass powder having a glass softening point in a range of about 400xc2x0 C. to 600xc2x0 C. The reason for using this type of glass powder is that when the glass softening point is lower than about 400xc2x0 C., diffusion of metal contained in the leads occurs, causing defects in the insulating layers (degradation of insulating properties), and when the glass softening point exceeds about 600xc2x0 C., the glass paste remains unsintered. The reason for setting the proportion of the low melting point glass powder to about 1 to 30 percent by weight is that when the proportion of the low melting point glass powder is less than about 1 weight percent, sintering is not performed satisfactorily and bubbles remain in the insulating layers causing insulation failure between layers of the multilayer circuit, and when the proportion of the low melting point glass powder exceeds about 30 percent by weight, patterns may be deformed after firing, and metal used in the lead may diffuse into the glass, causing insulation failure. When the proportion of the high melting point glass powder exceeds about 99 percent by weight, sintering is not performed satisfactorily and bubbles remain in the insulating layers, causing insulation failure between layers of the multilayer circuit.
When a high melting point glass powder which satisfies the conditions of the present invention, i.e., the high melting point glass powder having a glass softening point (TS) of about 300xc2x0 C. or more higher than that of the low melting point glass powder, is used, by firing at a temperature about 10xc2x0 C. to 100xc2x0 C. higher, preferably about 20xc2x0 C. to 30xc2x0 C. higher, than the glass softening point (TS) of the high melting point glass, the viscosity of the low melting point glass is appropriately decreased, promoting sintering. Thus, sintering can be performed easily and reliably. In addition, because the viscosity of the high melting point glass is not decreased as much at the time of firing, deformation in the shape of the pattern can be prevented.
It is to be noted that when the difference between the glass softening point of the low melting point glass and the glass softening point of the high melting point glass is less than about 300xc2x0 C., the low melting point glass does not flow properly, inhibiting sintering, even when the firing temperature is set to a temperature about 20xc2x0 C. to 30xc2x0 C. higher than the glass softening point of the high melting point glass. When the firing temperature is increased so that the low melting point glass may flow properly, the viscosity of the high melting point glass is reduced and the entire glass paste layer may flow, resulting in deformation of the pattern shape.
As described above, a low melting point glass having a glass softening point in a range of about 400xc2x0 C. to 600xc2x0 C. is preferably used as the low melting point glass. More preferably, a low melting point glass having a glass softening point in the range of about 450xc2x0 C. to 550xc2x0 C. is used.
Preferably, the photosensitive glass paste of the present invention includes a high melting point glass powder comprising an SiO2xe2x80x94B2O3xe2x80x94K2O type (borosilicate type) glass powder in which the proportion in weight of SiO2, B2O3 and K2O is in a region surrounded by point A (65, 35, 0), B (65, 25, 10), C (85, 5, 10) and D (85, 15, 0) in a ternary diagram shown in FIG. 1.
In the above described high melting point glass powder, the proportion of SiO2, which has a low reactivity with an organic binder, is high and the proportion of the composition (boron, in particular) having a high reactivity with an organic vehicle, particularly with a photosensitive organic binder having an acid functional group such as a carboxyl group, is relatively low. Thus, changes in viscosity of the photosensitive glass paste over time due to ionic cross-linking reactions are prevented, layers can be uniformly formed by using various techniques, and fine via holes, etc., having highly precise forms can be readily formed by a photolithographic technique.
An insulating body layer formed of a glass paste (insulating material) comprising a borosilicate glass of the above composition as the main component thereof displays superior characteristics, i.e., the relative dielectric constant ∈r is as low as 5 or less and insulation reliability in wet load testing is as high as 1xc3x97109 or more (log IRxe2x89xa79).
Furthermore, because a borosilicate glass capable of inhibiting the diffusion of a conductor component such as Ag is employed as the high melting point glass, it is possible to reduce the diffusion of metal (conductive component) used in leads and to decrease the conductivity of the glass layer. Thus, by using the photosensitive glass paste of the present invention, a multilayer electronic component, circuit or the like having less transmission loss can be formed.
By optimally varying the composition proportion of SiO2, B2O3 and K2O3 within the region surrounded by points A, B, C, and D in FIG. 1, it is possible to control the desired thermal expansion coefficient corresponding to the types of substrate material and conductive material within the range of, for example, 1.5 to 9 ppm/xc2x0 C. Accordingly, warpage of the substrate may be minimized, the deformation of the substrate may be decreased when layers are stacked and a thick film multilayered interconnected circuit board of high reliability can be obtained.
In region X shown in FIG. 1, the insulating resistance in wet load testing is lower and the insulation reliability is likely to decrease. In region Y shown in FIG. 1, the relative dielectric constant ∈r is high, and such a glass paste is not suitable for an insulating body layer of the thick multilayered interconnected circuit board for high frequency use. In region Z shown in FIG. 1, the sintering temperature for forming insulating layers is increased, and a low melting metal having a low specific resistance such as gold and copper will be difficult to fire simultaneously, thereby decreasing productivity.
Preferably, the composition of the SiO2xe2x80x94B2O3xe2x80x94K2O type glass is in the region surrounded by points E (75, 25, 0), F (75, 20, 5), G (85, 10, 5) and D (85, 15, 0) in FIG. 1.
More preferably, a photosensitive glass paste of the present invention includes a high melting point glass powder comprising a mixed powder containing the above-described SiO2xe2x80x94B2O3xe2x80x94K2O type glass and a SiO2xe2x80x94B2O3xe2x80x94Al2O3 type glass having a composition of about 93.5 to 97.8 percent by weight SiO2, about 2.0 to 5.0 percent by weight B2O3 and about 0.2 to 1.5 percent by weight Al2O3.
By using a mixture of a borosilicate glass having a composition within the region surrounded by points A, B, C and D in FIG. 1 and a SiO2xe2x80x94B2O3xe2x80x94Al2O3 type glass (high silica content silicate glass) comprising about 93.5 to 97.8 percent by weight SiO2, about 2.0 to 5.0 percent by weight B2O3 and about 0.2 to 1.5 percent by weight Al2O3, shrinkage of the paste due to firing can be inhibited and changes in the pattern shape due to firing can be minimized. Also, by using a high melting point glass having the above composition, viscous flow of glass is inhibited, via holes are prevented from being enlarged and a glass layer having a desired pattern may be formed on an insulating layer.
Also, by adjusting the proportions of the above-described borosilicate glass and the high silica content silicate glass, the thermal expansion coefficient of the insulating body layer and that of the substrate may be controlled to coincide so as to yield a multilayer circuit substrate with less warpage.
Preferably, the proportion of the high silica content silicate glass is adjusted to be about 15 to 35 percent by weight, and more preferably about 20 to 30 percent by weight, relative to 100 percent by weight of the borosilicate glass.
The above-described high silica content silicate glass has a high glass softening point (TS) and does not soften at the temperature at which the photosensitive glass paste, the product, is fired. The high silica content silicate glass not only functions as a filler for restraining shrinkage due to firing but also restrains the scattering of light and enhances the curing rate of the photosensitive glass paste because the refractive index thereof is close to the refractive index of the organic binder and the borosilicate glass.
Furthermore, because a high silica content silicate glass powder is easy to manufacture and the price thereof is stable, a high silica content silicate glass powder of reliable quality can be obtained at a lower cost.
The high silica content silicate glass powder has satisfactory wettability with the above-described borosilicate glass powder. Thus, sinterablilty can be enhanced and a densely sintered body can be formed.
When the above-described high silica content silicate glass has less SiO2 than that in the above region, the glass softening point will be lower and the refractive index will be higher. Thus, the difference in refractive index with other materials occurs in some part, scattering of light in those parts is increased, and the curing rate of the photosensitive glass paste may be undesirably lowered. When the content of SiO2 is higher than the above-described range, vitrification is difficult, resulting in increased cost.
Preferably, the photosensitive glass paste of the present invention includes about 40 to 70 percent by weight of the above-described inorganic component.
By setting the proportion of the inorganic component to the range of about 40 to 70 percent by weight, a photosensitive glass paste of superior quality can be obtained. That is, there is less shrinkage of the applied paste due to firing, voids are not significantly formed by firing, the insulation reliability of the formed layer is high and curing properties of layers are excellent.
When the inorganic component is less than about 40 percent by weight, it will be difficult to form a uniform layer due to the significant shrinkage by firing, and insulation reliability is degraded due to an increased number of voids. When the inorganic component exceeds about 70 percent by weight, scattering and absorption of light in the paste layers are increased at the time of development, causing the amount of light passing through the layers to be insufficient and a layer curing rate to decrease.
A method for manufacturing a multilayered interconnected circuit board of the present invention comprises: a step for printing the photosensitive glass paste on the insulating substrate having conductive leads formed thereon and drying the applied photosensitive glass paste; a step of forming a via hole pattern by exposing and developing the printed and dried photosensitive glass paste; and a step of forming an insulating layer with via holes by filling a conductive paste into the via hole pattern and firing the same.
Because the glass component which constitutes the main portion of the inorganic component of the photosensitive glass paste of the present invention contains a high melting point glass as the main component thereof and limits the proportion of the low melting point glass, by using this type of photosensitive glass paste, shrinkage due to firing can be prevented, diffusion of the conductive component such as Ag can be inhibited and a thick film multilayer circuit substrate of high reliability can be efficiently manufactured.
Furthermore, because the above-described manufacturing method employs the photosensitive glass paste of the present invention in order to form an insulating layer with via holes, shrinkage due to firing and diffusion of conductive component such as Ag can be inhibited. Thus, a thick film multilayered interconnected circuit board of high reliability can be efficiently manufactured.